1. Field of the Invention
This invention relates to a phase-locked oscillator and to a multi-radar system using the phase-locked oscillator. In particular, this invention is suitable for use in a phase-locked oscillator suitable for use in FM-CW radar or other RF-band oscillators, and for use in a multi-radar system using the phase-locked oscillator.
2. Description of the Related Art
FIG. 35A-35C are figures explaining technology of the prior art; FIG. 35A shows the frequency modulator of FM-CW radar of the prior art. In the basic configuration of the FM-CW method, a triangular-wave modulation signal is generated using a function generator (FG) or similar, and this modulation signal is used to apply frequency modulation to a voltage-controlled oscillator (VCO). Of importance in the FM-CW method is the application of precise triangular-wave frequency modulation; to this end, it is necessary, for example taking the center frequency as reference, that the maximum and minimum frequency deviation not change, and that the frequency change linearly with time, that is, that the slope thereof (rate of change of frequency) not change. The FM-CW radar output frequency depends on the stability of the VCO external conditions (temperature, power supply, and similar), and the frequency deviation of the output frequency depends on the VCO gain factor and output frequency, so that a VCO with high stability is necessary. Further, in order for the frequency to increase linearly, a VCO with good linearity is required.
FIG. 35B shows the typical configuration of an FM-CW radar transmitter of the prior art. To cope with changes with temperature in the oscillation frequency, the CPU uses the temperature detected by a temperature sensor to reference a data table, and corrects the center voltage of the triangular wave. With respect to linearity of the oscillation frequency, the CPU references the same data table to correct the triangular wave voltage.
FIG. 35C is a figure showing another configuration example of an FM-CW radar transmitter of the prior art, and shows a method of superposing a triangular wave in a PLL (Phase-Locked Loop) circuit. In this method, by phase-synchronizing the PLL with a center frequency, the center frequency is stabilized. On the other hand, with respect to the linearity of frequency deviation applied to this center frequency, a CPU corrects the triangular wave voltage by referencing a data table.
In the prior art, oscillation circuits are known in which, by using a triangular wave phase-synchronized with a crystal oscillator 6a as the modulation signal, the oscillation frequency is stabilized, and in addition, by frequency detection of the output RF signal, control is executed such that upper and lower limits of frequency deviation are not exceeded (Patent Reference 1).
Patent Reference 1: Japanese Patent Laid-open No. 6-120735
However, when a method is used to correct for temperature changes and linearity of the oscillation frequency using the above data table, not only is a separate large data table necessary, but it is necessary to prepare individual data tables for each device according to scattering in the circuit elements, and the number of testing processes is greatly increased. Further, in the configuration of the above FIG. 35C, because a PLL is used for feedback of the VCO output modulated by a triangular wave, the modulation characteristics of the VCO output are worsened. Also, the VCO modulation characteristics in Patent Reference 1 are assumed to be linear; if the characteristics are not linear, then a data table or similar must be used for linearity correction.
A multi-radar system comprises a plurality of individual arranged FM-CW radar units; by allocating each radar unit to a different detection area, a broader area can be detected with good precision. A multi-radar system may be a plurality of radar units installed in one device, or may be a plurality of devices with at least one radar unit installed. For example, when radar is installed in a vehicle such as an automobile, the radar installed in each vehicle forms a multi-radar system.
FIG. 36A-36B are figures explaining frequency allocation of each of the plurality of radar units in a multi-radar system of the prior art. FIG. 36A shows a case in which radio waves transmitted from radar units interfere; using the same frequency band, when triangular-wave modulated transmission signals are transmitted from a plurality of radar units, timing occurs in which transmission signals have the same frequency and it is impossible to determine from which radar unit a transmission signal was transmitted, so that accurate measurement becomes impossible. Consequently when a plurality of radar units are arranged, it is necessary to ensure that the frequencies of the signals transmitted from radar units do not mutually interfere, and so as shown in FIG. 36B, it is necessary to allocate to each radar unit at least a frequency band equal to the variation width of the frequency.
On the other hand, when the usable frequency band is limited, a broad frequency band cannot be allocated to a single radar unit. That is, the transmission signal frequency deviation cannot be made large. The larger the transmission signal frequency deviation, the higher is the sensitivity and resolution, and so when the allocated band is not sufficiently broad, the sensitivity and resolution are reduced.
Further, it is also necessary to leave a prescribed interval between adjacent transmission signals, taking into account changes in the center frequencies and frequency deviation of transmission signals due to changes in temperature, changes with aging, and similar. If these changes are large, then a large interval must be secured in order to prevent interference, and so the transmission signal frequency deviation magnitude or the number of radar units (number of channels) must be sacrificed.
FIG. 37A-37C are figures explaining changes in the center frequency and frequency deviation of transmission signals frequency-modulated by a triangular wave. FIG. 37A shows a case in which the center frequency of the transmission signals changes; FIG. 37B shows a case in which the transmission signal frequency deviation changes. FIG. 37C is a diagram showing an example of frequency allocation, taking frequency changes into account. In FIG. 37C, if in the 1 GHz band the frequency deviation is 200 MHz and the change is 50 MHz, then only two radar units can be positioned (only two channels can be secured), and the frequency band is not efficiently utilized.